The present invention relates generally to power circuits and systems and, more specifically, to power electronic switching devices.
Power electronic systems have many applications in today's technologies, such as motor drives and pulse power systems. As an example, consider the systems used to ignite turbine engines on aircraft, which systems are commonly referred to as exciters. Turbine engine ignition exciters have evolved considerably since their inception in the 1950's. First generation ignition exciters incorporated vacuum tube spark gap switching devices and vibrator type chopper, or DC-DC converter circuits. Given the severe operating environment of a turbine engine, i.e., high vibration, temperature extremes and widely varying input voltage and load impedance conditions, considerable effort was expended by the industry to develop highly reliable first generation components and circuit configurations.
Second generation ignition exciters were typically characterized by replacement of spark gap switching technology with a semiconductor switching device. These exciters commonly utilized three terminal (thyristor) switching devices to accomplish switching. This technology offered more precise spark timing and communication with an electronic engine control computer since feedback information could be effectively processed by the exciter charge pump (DC-DC converter) section. This provided a significant improvement over first generation devices, affording aviators considerably more control over the engine ignition process.
Conventional solid state ignition exciter circuits generally fall into one of two distinct categories. The first category being characterized by a series stack of switching devices to evenly distribute or divide the high (approximately 2-4 kV) tank capacitor voltage across each device. Such exciters incorporate elaborate voltage divider networks to protect and trigger the series ganged semiconductor switching devices from electrical imbalances generated during both the capacitor charge and discharge cycles. During the capacitor charge cycle, voltage must be evenly distributed across each device to prevent catastrophic failure of the entire series ganged semiconductor switching network. Likewise, during the discharge cycle, a series ganged back bias diode network is required to protect the switching devices from ring back which results during igniter plug firing. Circuits using this type of circuitry require careful screening and matching of thyristor leakage current and similar characteristics to ensure proper operation at elevated ambient temperatures. Moreover, this technology is electrically inefficient since the combined losses of several individual switching elements (during turn on and on-state) are very high.
By contrast, a second current circuit design philosophy incorporates a single thyristor switching device operated at a comparatively lower (0.4-1 kV) tank voltage. This approach offers considerably higher efficiency than the first design philosophy since only a single (VI) loss is incurred rather than multiple losses experienced with the aforementioned series ganged approach. However, this benefit is quickly offset since the tank capacitor voltage must be significantly lower than the series ganged approach to avoid using "hockey puck" type switching devices. While "hockey puck" type switches would offer adequate voltage ratings, their considerable bulk, weight and limited di/dt capability preclude their use in pulse discharge type circuitry. Therefore, current designs tend to utilize roughly 1,500 volt rated (phase control type) thyristors with correspondingly lower tank voltage.
To date, both types of solid state semiconductor switch exciter circuits have incorporated off-the-shelf, commercially available, phase control type thyristors. These devices, while readily available in commercial markets, are designed primarily for 60 Hz power control applications. Consequently, the dv/dt and di/dt ratings of these devices is limited with respect to requirements of a pulse power discharge application. The following summary provides a background of various thyristor technologies conventionally available for switch exciter circuits, as well as other power electronic applications.
Phase-Control Thyristors are designed to maximize the silicon for use as active emitter area at 60 Hz AC. The devices have large, shorted emitters (for high dv/dt) with single-point center gates, and depend on the relatively slow plasma spreading to turn on emitter areas remote from the point center gate. Researchers have extensively examined the spreading velocity by viewing radiative recombination of the plasma. Despite comprehensive research, spreading resistance of conventional devices necessitates low (e.g., less than 200 A/.mu.S) di/dt performance.
Inverter Thyristors have distributed or interdigitated gates (for high di/dt), similar to transistor emitter patterns, to turn on and utilize larger initial areas of the emitter for faster turn-on. For faster turn-off, heavy gold or platinum diffusion and/or electron radiation reduce carrier lifetime, thereby reducing thyristor turn-off times (tq). Unlike transistors, inverter thyristors have heavily shorted emitters to prevent latch-up when dv/dt is being applied. These inverter design features allow thyristors to be used at high (up to 10 kHz) repetition rates, but at the expense of high forward voltage drop. High forward drop severely limits performance of these devices in turbine ignition applications since increased (VI) power loss accelerates onset of thermal runaway, limiting upper temperature performance.
Gate Assist (GATO) and Gate Turn-Off (GTO) Thyristors have npn regions which are designed like high-speed transistors, where the gate (or base) is used for charge-control functions. In GATO closing switches, the gate is used to extract charge from the gate emitter junction during the tq and dv/dt switching interval. This allows high rep-rate performance without the adverse on-state voltage trade-off of lifetime-controlled inverter SCR's. The disadvantage of GATO's is the requirement for negative gate bias and current during the off-state and commutation interval, which considerably increases the complexity of exciter (thyristor) triggering circuitry. Gate Turn-Off Thyristors (GTO's) are similar to GATO's but must be lifetime controlled to act as opening switches. GTO's are made with both symmetric and asymmetric structures. Asymmetric GTO's are made both with and without anode shorts. The best turn-off gains for GTO's are obtained with shorted anode, asymmetric structures. So far, some of the highest di/dt pulse power closing switches have been GTO-type structures. These GTO emitter structures are ideally suited to receive and distribute high turn-on gating currents. If opening is not required, the highest possible hole-electron lifetimes will lead to the lowest possible on-state voltage. Therefore, GATO is perhaps the best conventional semiconductor switch structure for pulse-power applications such as turbine engine ignition systems.
MOS-Controlled Thyristors (MCT's) are integrated arrays of paralleled GTO cells (on the order of 20 micron spacing), with complementary FET's connected from anode to gate and gate to cathode. All of the cells have turn-off FET's that act as gate cathode shunts during turnoff and during the off-state. Some of the MCT cells have turn-on FET's connected from anode to gate. For those turn-on cells having their own anode-gate FET, the upper-base spreading resistance under the emitter is low, and good gate emitter injection is assured for good di/dt. However, not all cells have turn-on FET's and area utilization (60%) is not as good as with GATO's (&gt;85%), precluding use of devices in volumetric/weight sensitive aviation turbine engine applications. Furthermore, MCT gate-yield considerations limit the active area to about 1 cm.sup.2. High-current high-voltage applications are therefore better served by GATO-type designs, even though the turn-off function is not required.
As can be seen from the foregoing, thyristors have not yet been optimized to serve as both closing and opening switches for "square" reverse bias safe operating area (RBSOA) performance, i.e., turn-off to full current and voltage without snubbers. As a result designers have sought circuit solutions such as forced commutation for turn-off, resonant topologies (soft switching for low RBSOA switches), saturable reactors to limit di/dt, snubbers to limit reapplied dV/dt, and expensive gate drives to accommodate GTOs with low turn off gain and long turnoff "tail currents" due to open base recovery of the wide base pnp. The MCT has addressed many of these difficulties but also suffers from open base recovery (low RBSOA), is difficult to scale up to large areas with good yield, has a low active to total area ratio and a relatively small number of turn-on cells. Moreover, turn-on FETs associated with MCTs must block the full device voltage and must be sized based on the relationship blocking voltage to the 2.5 power, i.e., (blocking voltage).sup.2.5.
Thus, it would be desirable to provide the industry with power electronic switching devices which can serve as both closing and opening switches at unlimited di/dt's and without snubbers or saturable reactors.